Golf club head

ABSTRACT

A golf club having an increased moment of inertia and improved ball-hitting directionality is provided. A metal hollow golf club head comprises: a face portion; a crown portion; and a sole portion, and when the golf club has a lie angle of 60° with its club head volume being within 470 cm 3 , a moment of inertia about the axial line centered on the plumb line passing through the golf club head center of gravity is 5000 to 6000 g-cm 2 . In order to increase the moment of inertia, the thickness of the center portion of the crown portion is reduced by chemical etching, and a mass, including the portion of mass reduction, is positioned in the sole portion on the side of the toe portion; moreover, the separation distance from the center of gravity to the mass is increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a golf club head with improvedball-hitting directionality, and more specifically, relates to a golfclub head having a large moment of inertia and improved stability ofdirection of a hit ball.

2. Description of the Related Art

Various improvements have been made to golf clubs, to extend flightdistances, and to enable stable hitting of the ball. Flight distancesdirectly affect scores, and so through improvements so as to broaden theeffective range (“sweet” area) of the striking point on a golf club head(hereafter also simply called a “head”), improvements in the position ofthe effective area, improvements in the material of the face surface,and similar, the probabilistic ball flight distance has been extended;consequently scores have improved, and players using such heads havefound them beneficial. Moreover, in order to improve scores, golf clubshave been sought for which the direction of ball-hitting is stablydetermined even when there is deviation of the striking point. For thisreason, normally the moment of inertia must be made large. Inparticular, the lateral moment of inertia is an important factordetermining the direction of the ball.

This is because, if the moment of inertia is made large, when thestriking position at which the ball is struck is shifted, so that forexample the ball is struck on the toe side of the golf club head, theclub does not readily bend. That is, if the moment of inertia of thegolf club head is made large, then as explained above, even when theball is hit off-center there is little shake of the head, and the ballis driven in a comparatively straight direction. Hence the averageflight distance is extended, and as a result scores are improved.

The wood material (persimmon) of the woods used from long ago had atendency to cause the head to rotate easily when the ball was stuck; butmodern woods, made of metal and with a hollow interior, have a largermoment of inertia compared with wooden woods, so that there is littlerotation and no similar tendency, so that at present such clubs are usedby many players and have become the mainstream. These hollow metal-typewoods have grown in volume, but current rules stipulate a maximum volumeof 460 cm³ (with a tolerance of 10 cm³).

There is a trend toward larger heads, but the masses of the constituentparts of a head adds up, and the swing balance, which is a criterion forease of swinging of a club, becomes heavy. As a result, head masses havein the prior art been no greater than approximately 210 g. That is,given the configuration of the prior art, although heads have tended toincrease in size the total mass has been limited, and so excess mass tocontrol the position of the center of gravity, or in other words, excessmass to increase the moment of inertia has in the prior art been limitedto approximately 10 g, due to the constraint that the total mass shouldnot be increased.

According to R&A rules, the pendulum test method is adopted to measurethe restitution coefficient of the face surface. This testing methodentails fixing the club, causing a steel sphere to collide with the facesurface, and measuring the contact time; the contact time is called thecharacteristic time, and the rule limits this characteristic time to 257μs (microseconds) or less (including a tolerance of 18 μs). In order tokeep this characteristic time at or less than the time stipulated by therules, the thickness of the face portion sheet tends to become thick,but there is a limit to the extent to which the face portion mass can bereduced. Further, the hosel portion connected to the shaft is positionedon one end of the face portion, and the weight of this portion is alsorelatively great.

Further, the above-described rules also impose various constraints onexternal dimensions, such as that the length from the heel portion tothe toe portion must not be longer than the length from the face portionto the rear surface; that the length from the heel to the toe must be127 mm (5 inches) or less; and that the length from the sole to thecrown must be 71.12 mm (2.8 inches) or less. And, there is the furtherconstraint that the volume must be 470 cm³ (including a tolerance of 10cm³) or less. Given these constraints as well, the center of gravityposition cannot be located in a position so as to increase the “sweet”area, and moreover the overall head mass becomes large. Due to suchconstraints, it is extremely difficult to increase the weight or otherexcess mass so as to increase the moment of inertia.

Despite such engineering difficulties, various proposals to increase themoment of inertia have been made. For example, heads are known in whichmass is distributed in at least one direction among the three majorinertial axes in orthogonal coordinates passing through the center ofgravity, or in sites in proximity thereto, or with masses distributed insuch a manner (see for example Japanese Patent Laid-open No. 5-57034).And, clubs are known in which the golf club head comprises metalmaterial members and fiber-reinforced resin members, with the headbonded together by an adhesive of thickness 0.05 to 1 mm (see forexample Japanese Patent Laid-open No. 2003-320060). Also, technology isknown in which an aperture portion is provided in the crown portion, anda fiber-reinforced resin with specific gravity smaller than metalmaterials is used in this aperture portion, to improve the ball-hittingdirectionality (see for example Japanese Patent Laid-open No.2005-278838).

Thus various measures have been taken to extend the flight distance ofgolf clubs, but at present club performance remains not entirelysatisfactory. While the technologies described above have representedpartial improvements, problems remain, and there is still room forimprovement. Metal hollow golf club heads tend to increase in size, asdescribed above, and if the moment of inertia is increased, the volumetends to increase as well; if the volume is increased while makingefforts to limit mass, strength-related problems arise; and so therehave been limits to the methods employed in the prior art. When forexample using fiber-reinforced resins as described above, not only dostrength-related limits appear, but there are the problems ofunsatisfactory ball-hitting sounds and resistance to damage. On theother hand, insofar as is known by these inventors, there exist no golfclubs in the prior art, primarily comprising metal members, with amoment of inertia in the range 5000 to 6000 g-cm².

This invention was devised in order to resolve the above-describedproblems of the prior art, and attains the following objects.

SUMMARY OF THE INVENTION

An object of the invention is to provide a golf club with an increasedmoment of inertia of a metal hollow golf club head with large volume,and with improved ball-hitting directionality.

A further object of the invention is to provide a golf club with anincreased moment of inertia of a metal hollow golf club head with largevolume without increasing the head mass, and with improved ball-hittingdirectionality.

In order to attain the above objects, the following means are employed.

The golf club of Invention 1 is a golf club having a metal hollow golfclub head, comprising: a face portion positioned on a front surface ofthe metal hollow golf club head and having a striking face to strike agolf ball; a crown portion forming an upper surface of the club, and asole portion forming a lower surface of the club, characterized in thatthe mass of the metal hollow golf club head is 210 g or less,characteristic time (CT value) of the metal hollow golf club head,relating to a restitution characteristic, is 257 μs or less; and when alie angle of the metal hollow golf club head is 60°, volume of the metalhollow golf club head is 470 cm³ or less, and moment of inertia about anaxial line which is the center of a plumb line passing through thecenter of gravity of the metal hollow golf club head, is in the range5000 to 6000 g-cm².

The golf club of Invention 2 is the golf club of Invention 1,characterized in that the metal is a titanium alloy sheet member, and asubstantial center portion and an outer peripheral portion includingsites of the plumb line within the curved surface of the crown portionand/or the sole portion constituting the body differ in thickness.

The golf club of Invention 3 is the golf club of Invention 1 orInvention 2, characterized in that the metal hollow golf club head isformed by joining, by welding, the face portion, the sole portion, thecrown portion, and a hosel portion to which the shaft is connected.

The golf club of Invention 4 is the golf club of Invention 2,characterized in that a weight of 20 g or more is positioned at aposition of a rotation radius most distant from the plumb line and atthe crown portion and/or the sole portion.

The golf club of Invention 5 is the golf club of Invention 4,characterized in that, in the golf club of Invention 4, the weight ispositioned on the back side of the toe portion.

The golf club of Invention 6 is the golf club of Invention 5,characterized in that the toe-side and back-side sites of the soleportion are formed in shapes protruding outward relative to the crownportion, and that the weight is positioned in this protruding soleportion.

The golf club of Invention 7 is the golf club of Invention 1,characterized in that the metal hollow golf club head has:

a length from the heel portion to the toe portion longer than the lengthfrom the face portion to the rear surface;

a length from the heel to the toe of 127 mm (5 inches) or less;

a length from the sole to the crown of 71.12 mm (2.8 inches) or less;and,

a moment of inertia (MOI) within the range calculated using thefollowing approximating equation (1):

MOI=(aY ² +bY+c)×(dX+e)/f  (1)

where X is the length from the heel portion to the toe portion, Y is thelength from the face portion to the rear surface, and a, b, c, d, e, andf are constants.

The golf club of Invention 8 is the golf club of Invention 1,characterized in that the metal hollow golf club head has:

a length from the heel portion to the toe portion longer than the lengthfrom the face portion to the rear surface;

a length from the heel to the toe of 127 mm (5 inches) or less;

a length from the sole to the crown of 71.12 mm (2.8 inches) or less;and,

a position of the center of gravity, as seen from the plumb direction,existing within the range encompassed by the following two equations:

Y=−gX ² +hX ² +i  (2)

Y=−jX ² +k  (3)

where X is the position in the direction from the heel portion to thetoe portion, Y is the position in the direction from the face portion tothe rear surface, g, h, j, and k are constants, X has its origin at thecenter of the distance from the heel portion to the toe portion, and Yhas its origin in the face portion.

As explained in detail above, a golf club of this invention employs ahollow golf club head, the materials comprised by which are in essenceall metals; under the constraints that the head volume be 470 cm³ orless (including a tolerance of 10 cm³), that the head mass be 210 g orless, and that the head characteristic time (CT value) related to therestitution characteristic be 257 μs or less (including a tolerance of18 μs), a method and conditions were discovered for obtaining a golfclub head for which the moment of inertia about the axial line centeredon the plumb line passing through the center of gravity of the metalhollow golf club head is in the high range 5000 to 6000 g-cm². As aresult, even when the striking point deviates from the center of theface portion, the direction in which the ball is hit is secured, andstable striking is possible compared with clubs of the prior art, sothat as a result there is a strong possibility of improvement of aplayer's score. Further, because in essence the club head is of metal,satisfactory performance with respect to durability, ball-hitting sound,and other aspects can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view showing the overall configuration of a golfclub;

FIG. 2 is a plane view of Aspect 1 of a driver club head;

FIG. 3 is a front view of Aspect 1 of a driver club head;

FIG. 4 is a side view of Aspect 1 of a driver club head;

FIG. 5 is a cross-sectional view along X-X in FIG. 4;

FIG. 6 is a plane view of Aspect 2 of a driver club head;

FIG. 7 is a plane view of Aspect 3 of a driver club head;

FIG. 8 is a front view of Aspect 3 of a driver club head;

FIG. 9 is a side view of Aspect 3 of a driver club head;

FIG. 10 arranges the external shapes of the heads 1 for which the momentof inertia MOI is calculated;

FIG. 11 shows the relation between head width and moment of inertia,when the toe-heel length is held constant;

FIG. 12 shows the relation between toe-heel length and moment of inertiawhen the head width is held constant;

FIG. 13 is a graph showing the relation between toe-heel length and headwidth for different moments of inertia;

FIG. 14 is an example in which the position of weight placement in ahead 1 is changed; and,

FIG. 15 shows the relation between moment of inertia and position ofcenter of gravity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspect 1

Aspect 1 of the invention is explained referring to the drawings. FIG. 1is an external view of an entire golf club of this invention, and showsthe driver club head having a metal hollow golf club head. A golf clubof this invention employs a metal hollow golf club head; the driver clubhead (hereafter also called a “head”) of Aspect 1 similarly is a metalhollow golf club head. The basic construction of a driver club head iswell-known, and a detailed explanation is omitted; however, in order tofacilitate understanding of the invention, a summary explanation isgiven as follows.

A driver club head 1 of this invention has one end of the shaft A fixed.FIG. 2 to FIG. 4 show an aspect of a driver club head 1 of a metalhollow golf club of this invention. FIG. 2 to FIG. 6 show only the headportion; the shaft A and other members are not related to the gist ofthis invention, and so are omitted from the drawings.

FIG. 2 is a plane view of the head 1, FIG. 3 is a front view of the head1, and FIG. 4 is a side view of the head 1. As shown in each of thedrawings, the metal hollow driver club head 1 comprises a crown portion2, which is the upper portion; a sole portion 3, which is the bottomportion; a face portion 4, which strikes the golf ball; a toe portion 5,which is the front portion of the head 1; a heel portion 6, which is therear portion of the head; a back portion 10, positioned on the oppositeside from the face portion 4, forming the rear portion of the head 1;and a hosel portion 7, which is a member which supports and fixes thedriver club head 1 on the shaft A. Among these portions, the principalportions are the face portion 4, crown portion 2, sole portion 3, andhosel portion 7.

For manufacturing reasons, each of these portions comprises either oneor a plurality of sheet members, which are joined together. Inmanufacturing each portion, sheet members are press-molded to thedesired curved-surface shape, and are then integrated by welding orother means. Rolled members are particularly suitable for use as sheetmembers for purposes of controlling the sheet thickness. In thisexample, the body member comprising the head 1 is formed by combiningfour members, which are a face member; a sole member, comprising a toeportion 5, heel portion 6, and a portion of the back portion 10; a crownmember, comprising a toe portion 5, heel portion 6, and a portion of theback portion 10; and a hosel member.

Each of the four members is formed by cutting the sheet members intoprescribed shapes and then heating and pressing. The face member forexample is heated to 400°, and the sole member, crown member, and otherbody member were heated to 900°. After pressing, burrs were cut away(trimming), and TIG welding was performed. TIG welding is a type ofwelding also called argon welding; a welding rod of the deposit metalitself is used, and argon gas is released from the periphery of atungsten electrode, to shield the molten metal from the atmosphereduring welding. Also, laser and plasma welding methods may be used, withfewer welding beads or thermal effects, making such methods moreappropriate. Each of the members may also be manufactured by casting,forging, or another method.

In this Aspect 1, the metal material is a titanium alloy, the facemember and sole member are opposed as members in manufacturing, andthereafter the hosel member is joined, and then the pressed crown memberis bonded by TIG welding or similar. In this way, an integrated driverclub head 1 is formed by welding. The face portion 4 has a minutelycurvature surface, and is formed in a plate shape. The area of maximumrestitution coefficient is the center portion which is the strikingface, that is, the “sweet” area 9 near the center of gravity 8.

Normally, in order to send the golf ball a great distance, it iseffective to strike the ball at this “sweet” area 9, corresponding to aposition near the center of gravity 8; to this end, this area is madelarger and the area with a high restitution coefficient is expanded, or,the restitution coefficient is set to a high value to enhance therestitution effect. As explained above, it is well known that if therestitution coefficient is high, the golf ball will travel a greatdistance, and this restitution coefficient constitutes an importantparameter of golf club performance, so that as stated above measurementcriteria are stipulated by the U.S. Golf Association (USGA), R&A rules,and other authorities. Titanium alloys often used in driver club heads 1include β-type titanium alloys and α+β-type titanium alloys. Thesealloys have enhanced strength as well as excellent machinability,ductility, toughness, and strength, and are reliable alloys.

In this Aspect 1, the basic driver club head 1 comprised by such a golfclub has improvements added to the crown portion 2 and sole portion 3 inparticular, in order to increase the moment of inertia. The moment ofinertia is represented by m×r²; hence it is clear to a practitioner ofthe art that it is sufficient to increase either m (the mass of the head1) or r (the distance from the center of gravity). However, the mass ofthe general head overall is limited to approximately 210 g due to thefact that, as described above, if the head is made too heavy the swingbalance suffers. In particular, due to the larger volumes of recentheads 1, the mass of the head 1 cannot be made too great.

Hence given these constraints, it is difficult to increase the mass ofthe head 1. In this aspect, by increasing the distance from the centerof gravity (r) to the extent allowed by the relation to the mass (m) ofthe head 1, the moment of inertia is increased. In particular, when thelie angle of a metal hollow golf club head is set to 60°, the moment ofinertia about the axial line centered on the plumb line passing throughthe center of gravity of the head 1 is increased. Below, the headconstruction by which this was achieved is explained.

FIG. 5 is a cross-sectional view along X-X in FIG. 4. Thecross-sectional view is in the same direction as FIG. 3, passing throughthe center of gravity 8. In the figure, the symbol B is the distancefrom the center of gravity 8 to the hosel portion 7, that is, thecenter-of-gravity distance. The mass and shape of the hosel portion 7 ineffect cannot be changed, due to the circumstances of mounting of theprescribed shaft A. Hence in order to increase the moment of inertia, itis effective to increase the distance at which a mass is positioned fromthe center of gravity 8, that is, the separation distance C, and toincrease the mass. Hence in this Aspect 1, by changing the placementposition the separation distance C is increased. As explained above, thecrown portion 2 is formed by pressing of a sheet-shape titanium alloymember; this press-machined member is formed into an even thinner shape.In this crown portion 2, the thickness in the area D on the periphery ofthe center of gravity above the center of gravity 8 is made thin. Thatis, when the lie angle of the head 1 is set to 60°, thiscenter-of-gravity periphery area D is the area on the reverse face ofthe crown portion 2 centered on the plumb line (not shown) passingthrough the center of gravity 8 of the head 1.

The machining method to reduce the thickness of this center-of-gravityperiphery area D is, in this example, chemical etching treatment. Thischemical etching treatment is well-known, and a detailed explanation isomitted. Chemical processing is used to reduce the thickness in thecenter-of-gravity periphery area D of the crown portion 2, to furtherreduce the thickness of the crown portion 2. As the machining methodused to reduce the thickness of the crown portion 2, cutting and coiningby press machining are also possible to some degree; but use of suchmachining methods is limited by the machining hardness of the materialsand by various machining-related constraints, and thicknesses beyond acertain limit cannot be attained. Through this chemical etchingtreatment, the thickness of the center-of-gravity periphery area D canbe reduced to the desired value. By this means, titanium alloy alone canbe used to maintain strength, rather than opening a hole in the crownportion and covering with a fiber-reinforced resin instead of a metalmaterial in a composite configuration, as in the prior art.

Cutting-away of the center-of-gravity periphery area D of the crownportion 2 results in reduced mass of the crown portion 2, andconsequently the mass at sites relatively distant from thecenter-of-gravity periphery D is increased. Further, as shown in FIG. 5and FIG. 6, the portion in which cutting-away is avoided is the side ofthe toe portion 5 opposed to the hosel portion 7 from the center ofgravity 8. More rigorously, the position of the boundary portion on theside of the toe portion 5 and back portion 10 (see FIG. 2) ispreferable. This portion can be made distant from the center of gravity8, and has the greatest effect in increasing the moment of inertia.Moreover, it is preferable that further mass be placed at this portion,that is, at the end position at the separation distance C.

In this Aspect 1, as explained above, this is accomplished by reducingthe mass of the center-of-gravity periphery area D and leaving athickness 2 a at the end position at the separation distance C in thecrown portion 2. This results in the occurrence of a relative differencein mass between the center-of-gravity periphery area D and the endportion at the separation distance C. As a result, if the overall massof the head 1 does not change, this is equivalent to providing a mass atthe position of the end portion at the separation distance C. Moreover,the separation distance C was increased to the extent possible withinthe limitations of the overall mass. This was accomplished by enlargingthe grown portion 2 b on the side of the toe portion 5, and forming asubstantially rectangular shape. The shape approximates a so-calledsquare wood. This result was accomplished by reducing the thickness ofthe center-of-gravity periphery area D within the limitation of theoverall mass.

Next, an excess mass 3 a is provided at the sole end portion position atseparation distance C in the sole portion 3. This is a means to directlyincrease the mass, but in contrast with masses provided with the aim ofmore effective striking as in the prior art, the position of placementof the mass is limited, and the mass is provided at the site farthestremoved from the position of the center of gravity 8, that is, at thesite positioned at the separation distance C. The mass increase due tothis excess mass 3 a is within the limitation on overall mass, but tothe extent that the mass of the center-of-gravity periphery D of thecrown portion 2 is cut away, this mass can be added, so that greatermass increase is possible than in the prior art. For example, whereas inthe construction of a conventional head an added mass was approximately10 g, in this aspect 1, addition of a mass of 20 to 25 g is possible. Anadded weight does not only take the form of provision of a separatebody, but also includes weight added by increasing the thickness of aplate member itself.

In this way, in this Aspect 1, a portion of the crown portion 2 isremoved by chemical etching, and a mass 3 a is provided in the soleportion 3, to increase the value of the mass m to the extent possible.As described above, with respect to the numerical value of the distancer, by forming the side of the extended crown 3 b in particular into anenlarged rectangular shape, the separation distance C of the mass 3 afrom the center of gravity 8 can be increased. This separation distancefrom the center of gravity 8 is indicated by C in FIG. 5; in FIG. 2, theextended crown portion 3 b is in a position on the side opposite thesubstantially diagonally line opposing the center-of-gravity distance Bon the side of the hosel portion 7, and has an enlarged shape comparedwith the extended crown portion 3 b in the position of the prior art. Asa result, through the multiplied effect of the increased numericalvalues of the distance (r) from the center of gravity 8 and of the mass(m) of the head 1, a moment of inertia of 5000 to 6000 g-cm² is achievedin a head with an increased volume of 470 cm³.

Aspect 2

FIG. 6 is a plane view showing Aspect 2 of the invention. In thisexample, relative to the crown portion 2, the end portion 3 b of thesole portion 3 on the side of the toe portion 5 and back portion 10 isenlarged, and a mass 3 a is positioned. Other portions of this head 1are effectively the same as in Aspect 1, and explanations are omitted.In this Aspect 2, the numerical value of the distance r equivalent tothe separation distance C is further increased, so that the moment ofinertia can be further increased. Moreover, when examining the moment ofinertia of a metal hollow golf club head, the lie angle is set to 60°.The lie angle is the angle bade by the face of the head in contact withthe ground and the shaft; in the case of a metal hollow golf club head,even when the head is enlarged with increased volume and large moment ofinertia, experiments have indicated that a lie angle of approximately60° is most satisfactory.

Making the moment of inertia large in this way means that, even whencontact with the ball deviates from the center, or in other words is onthe side of the toe portion 5 from the “sweet” area 9 of the faceportion 4, or in an extreme case the ball is mis-struck, becausevibration of the head 1 does not readily occur there is little bendingof the hit ball compared with the prior art, and the direction of thehit ball is stable compared with a case of a smaller moment of inertia.

Aspect 3

FIG. 7 to FIG. 9 show Aspect 3 of a driver club head; FIG. 7 is a planeview of Aspect 3 of a driver club head, FIG. 8 is a front view of Aspect3 of a driver club head, and FIG. 9 is a side view of Aspect 3 of adriver club head. The external shape of this head 1 is substantiallyquadrilateral as seen in plane view, with the angle in one corner formedinto an arc, as represented characteristically in the plane view of FIG.7. The metal hollow driver club head 1 of this Aspect 3 is createdentirely from titanium alloy sheet. The body members comprised by thehead 1 of this Aspect 3 are the four members which are the face portion4, sole portion 3, crown portion 2, and hosel portion 7. The crownportion 2 comprises titanium alloy sheet (specific gravity 4.51) ofuniform thickness 0.5 mm. The sole portion 3 comprises titanium alloysheet (specific gravity 4.51) of uniform thickness 0.75 mm. Similarly,the hosel portion 7 and weight 3 a comprise titanium alloy (specificgravity 4.51).

The face portion 4 is entirely formed from titanium alloy sheet(specific gravity 4.42); the thickness is different in differentportions. In the ellipse-shaped center portion 4 a, the thickness is 3.1mm. In the outer portion 4 b on the outside of the center portion 4 a,the thickness is 2.3 mm. In this way, the thickness differs in differentportions of the face portion 4 in order to cause the characteristic time(CT value) of the restitution coefficient of the face portion 4 to bethe stipulated 257 μs or less (including a tolerance of 18 μs), and sothat the mass of the face portion 4 is not increased. The flange 4 c onthe outer periphery of the face portion 4 is of 1.3 mm thick titaniumalloy sheet (specific gravity 4.42) with constant width. The flange 4 cis also placed at the positions of the crown portion 2 and sole portion3. This flange 4 c supports the face portion 4 on the outer periphery,and in addition functions to link the crown portion 2 and sole portion3, which are thin, and the face portion 4. The construction of the hosel7 has a generally employed shape, and the shape is not special, so thata detailed explanation is omitted.

Moment of Inertia MOI

On both sides of the back portion 10 is placed a weight 3 a. As shown inFIG. 7, more of the weight 3 a is placed at the corner portions on bothends. This is in order to increase the distance from the center ofgravity 8 and increase the moment of inertia MOI. As shown in FIG. 8,the distance from the end of the toe portion 5 of the head 1 to a pointin the heel portion 5 at a height of 22.23 mm (0.875 inches) from thebottom face of the sole portion 3 is called the “toe-heel length X”. Asshown in FIG. 9, the length from the end of the face portion 4 to theend of the back 10 is called the “head width Y”. Given the constructionindicated for this Aspect 3, the moment of inertia MOI about the plumbline passing through the center of gravity 8 was calculated. The methodof calculation was as follows. Taking as basic the head 1 of this Aspect3, the external dimensions were systematically varied, and the moment ofinertia MOI calculated for each case.

FIG. 10 arranges the external shapes of heads 1 for which the moment ofinertia MOI was calculated. Heads 1 arranged in this way haveincreasingly larger “toe-heel lengths X” in moving rightward along thehorizontal axis (in the figure) (87 to 127 mm), and have increasinglylarger “head widths Y” in moving upward along the vertical axis (in thefigure) (86 mm to 126 mm). These heads 1 have a hosel portion 7 with thesame shape and dimensions as in Aspect 3; the flange width dimension ofthe outer periphery 4 c of the face portion 4 is also the same, and asshown in FIG. 10, only the head width Y and toe-heel length X werevaried. However, rules for golf equipment stipulate that the “toe-heellength X” must not be longer than the “head width Y”. Hence heads 1 inthe upper-left in FIG. 10 cannot be used, and so calculations wereomitted.

The heads 1 shown in the first column (right-hand column) in FIG. 10have a “toe-heel length X” fixed at 127 mm, and “head widths Y” of 126mm, 116 mm, 106 mm, 96 mm, and 86 mm. The moment of inertia MOI wascalculated for each of these as described below. The head width Y (mm),head volume (cm³), moment of inertia MOI (g-cm²), and mass of the weight3 a (g), appear in Table 1. The overall masses for heads 1 were 205 g ineach case. Hence as shown in Table 1, the volumes, and the masses ofweights differed for each of the heads 1. The moment of inertia aboutthe axial line (lateral) centered on the plumb line passing through thecenter of gravity of the head 1, which has been an object of thisinvention, was in the range 5000 to 6000 g-cm². As is seen from the dataof Table 1, for a “head width Y” of 126 mm, 116 mm, and 106 mm, a momentof inertia of approximately not less than 5000 g-cm², which was anobject of this invention, was exceeded.

TABLE 1 Head Moment of Weight Width(mm) Volume(cm³) inertia(g-cm²)mass(g) 126 460.8 5,900 43.91 116 424.6 5,424 51.23 106 388.3 4,96758.40 96 352.0 4,550 65.68 86 315.7 4,181 72.91 Here, the “toe-heellength X” is 127 mm, and the head mass is 205 g.

Similarly, the heads 1 in the second column from the right in FIG. 10have the “toe-heel length X” fixed at 117 mm, and “head widths Y” of 126mm, 116 mm, 106 mm, 96 mm, and 86 mm; the moments of inertia MOI foreach of these were calculated. The data appears in Table 2. As is seenfrom this Table 2, when the “head width Y” is 126 mm or 116 mm, thedesired moment of inertia of 5000 g-cm² or higher is achieved.

TABLE 2 Head Moment of Weight Width(mm) Volume(cm³) inertia(g-cm²)mass(g) 126 424.6 5,581 53.05 116 391.1 5,046 60.01 106 357.7 4,53766.66 96 324.3 4.074 73.48 86 290.8 3.664 80.13 Here, the “toe-heellength X” is 117 mm, and the head mass is 205 g.

FIG. 11 shows the relation between head width and moment of inertia,when the toe-heel length is held constant. That is, FIG. 11 plots theresults of Tables 1 and 2 with the head width Y along the horizontalaxis and the moment of inertia MOI along the vertical axis; the momentof inertia MOI is plotted along the vertical axis with the “toe-heellength X” dimension fixed. This FIG. 11 plots the moments of inertia MOIof the heads 1 with the construction of Aspect 3, holding the “toe-heellength X” constant. From this data, heads with a moment of inertia MOIexceeding 5000 g-cm², which is an object of this invention, areapproximately three in number when the “head width Y” is 126 mm, and areapproximately two in number when the “head width Y” is 116 mm.Probabilistically, the moment of inertia is near 5000 g-cm² when the“toe-heel length X” is 127 mm, and so this data is adopted as anapproximating equation. When in FIG. 11 each of the points of the momentof inertia for which the “toe-heel length X” is 127 mm are connected,the resulting figure is a quadratic curve. An approximating equation forthis quadratic curve is equation (3) below, and so when the “toe-heellength X” is fixed, this approximating equation is adopted in thevicinity of 5000 g-cm².

MOI=0.18Y ²+4.191Y+2437.7  (3)

Similarly, heads 1 in the first row from the top in FIG. 10 have the“head width Y” fixed at 126 mm, and the “toe-heel length X” at 127 mmand 117 mm. However, a “toe-heel length X” of 117 mm violates the rules,and so was removed from consideration. Similarly, heads appearing in thesecond row in FIG. 10 have the “head width Y” fixed at 116 mm, and a“toe-heel length X” of 127 mm and 117 mm. Similarly, heads appearing inthe third row in FIG. 10 have the “head width Y” fixed at 106 mm, and a“toe-heel length X” of 127 mm, 117 mm, and 107 mm. Similarly, headsappearing in the fourth row of FIG. 10 have the “head width Y” fixed at96 mm, and a “toe-heel length X” of 127 mm, 117 mm, 107 mm, and 97 mm.Similarly, heads appearing in the fifth row of FIG. 10 have the “headwidth Y” fixed at 86 mm, and a “toe-heel length X” of 127 mm, 117 mm,107 mm, 97 mm, and 87 mm. In this way, moments of inertia MOI of theheads were calculated with the “head width Y” dimension fixed.

FIG. 12 shows the relation between the toe-heel length and the moment ofinertia when the head width is held constant. That is, FIG. 12 plots the“toe-heel length X” along the horizontal axis and the moment of inertiaMOI along the vertical axis with the “head width Y” dimension fixed.From this data, heads with a moment of inertia exceeding 5000 g-cm²,which is an object of this invention, are only heads with the head widthY fixed at 106 mm. The line connecting these points is a straight line,an approximating equation for which is given by equation (4); thisequation (4) is adopted as an approximating equation in the vicinity of5000 g-cm².

MOI=37.84X+618.3  (4)

In the vicinity of a moment of inertia of 5000 g-cm² or higher, as therelation between the toe-heel length (X) and head width (Y), these twoapproximating equations (3) and (4) are combined to obtain the followingapproximating equation (5) in the vicinity of 5000 g-cm².

MOI=(0.181Y ²+4.191Y+2437.7)×((37.84X+618.3)/5424)  (5)

This equation (5) is a product of the approximating equation (3) and theapproximating equation (4); by taking the product of the twoapproximating equations, a relation is derived between the moment ofinertia in the vicinity of 5000 g-cm², the head length X, and the headwidth Y. Here the numerical value “5424” is the moment of inertia whenthe “toe-heel length X” in approximating equation (4) is 127 mm.Approximating equation (5) is the product of approximating equation (3)and approximating equation (4), and so is divided by the moment ofinertia calculated using approximating equation (3) in the vicinity of5000 g-cm². By this means, the value of the moment of inertia ofapproximating equation (5) is corrected.

As is understood from the above explanation, the numerical values inapproximating equation (5) are intrinsic numerical values arising fromthe shape, construction, materials, masses, and similar of the headspecific to this Aspect 3, and so these values can be replaced withconstants. That is, the approximating equation (5) can be represented asthe following general equation.

MOI=(aY ² +bY+c)×(dX+e)/f  (1)

Here X is the length from the heel portion to the toe portion, Y is thelength from the face portion to the rear surface, and a, b, c, d, e, andf are constants.

FIG. 13 is a graph showing the relation between the toe-heel length andthe head width for different moments of inertia. FIG. 13 is a generalsummary of the above explanation; here the horizontal axis indicates X(toe-heel length) and the vertical axis indicates Y (head width). InFIG. 12, values above the diagonal line have a head width greater thanthe toe-heel length, and violate the rules, and so describe heads whichcannot be commercialized. The area below the diagonal line describesheads which comply with the rules, and represents heads having a lateralmoment of inertia of 5000 g-cm² or higher, an object of this invention,which have been manufactured and commercialized.

The above-described approximating equation (5) was calculated for eachof the moments of inertia 5000, 5200, 5400, 5600, 5800, and 5900 g-cm².From FIG. 13, if a golf club head having a moment of inertia in therange 5000 to 5900 g-cm², which is an object of this invention, is to beobtained, the toe-heel length and head width of a head with the shape ofthat of Aspect 3 can be determined. Further, as a result, the weightmagnitude can also be determined.

Center of Gravity Position

Next, differences in the moment of inertia with the center of gravityposition of the head of Aspect 3 are explained. In FIG. 14A to 14E areexamples in which the position of placement of a weight 3 a in the head1 is varied. The specifications and thicknesses of the crown portion 2,sole portion 3 and face portion 4 are as described above. Further, whenthe specifications of this head 1 include a “toe-heel length X” of 127mm, “head width Y” of 126 mm, volume of 460 cm³, and head mass of 205 g,the weight mass is 43.9 g. FIG. 15 shows the relation between moment ofinertia and center of gravity position for heads with thesespecifications. The horizontal axis X in FIG. 15 indicates theX-direction center of gravity position, in the toe-heel lengthdirection. The vertical axis Y indicates the Y-direction center ofgravity position, in the head width direction. The origin (O) of thehorizontal axis X is the center position of 127 mm. The origin (O) ofthe vertical axis Y is the surface of the face portion 4. Hence FIG. 15is equivalent to a plane view of the head 1.

The curve in FIG. 15 for a moment of inertia of 5000 g-cm² is the resultof using the above-described approximating equation (5) to plot thecenter of gravity in the vicinity of a moment of inertia of 5000 g-cm².This curve is the result of fixing the overall mass of the head 1 at 205g, the mass of weight 3 a at 43.91 g, the toe-heel length at 127 mm, thehead width at 126 mm, and the volume at 460 cm³, so that the moment ofinertia is 5000 g-cm², and representing the center of gravity positionwhen the position of the weight 3 a is varied as shown in FIG. 14. Themass of the weight 3 a is determined when the overall mass of the head 1is 205 g and the volume is constant at 460 cm³. At this time, the centerof gravity position of head 1 is changing, and the moment of inertia ischanging, when the position of the weight 3 a is varied as in FIG. 13,the maximum moment of inertia area, and the area in which the moment ofinertia is 5000 g-cm², can also be calculated. This maximum moment ofinertia area and area resulting in a moment of inertia of 5000 g-cm² areindicated by line segments.

The maximum moment of inertia curve and the 5000 g-cm² moment of inertiacurve are represented by the following approximating equations (6) and(7).

Y=−0.0668X ²+0.1318X+56.66  (6)

Y=−0.1558X ²+0.6363X+41.53  (7)

Hence one condition to obtain a head with a moment of inertia of 5000g-cm² or above is that the center of gravity position be set in the areaenclosed between these approximating equations (6) and (7). The positionand size of the “sweet” area of the face portion 4 are also related tothis center of gravity position, and so are important.

As is understood from the above explanation, each of the numericalvalues of the approximating equations (6) and (7) are intrinsicnumerical values arising from the shape, construction, materials,masses, and similar of the head specific to this Aspect, and so thesevalues can be replaced with constants. That is, the approximatingequation (5) can be represented as the following general equations.

Y=gX ² +hX ² +i  (2)

Y=jX ² +k  (3)

Here, X is the position from the heel portion in the toe portiondirection, Y is the position from the face surface in the rear facedirection, and g, h, j, and k are constants; the origin of X is taken tobe the center of the length from the heel portion to the toe portion,and the origin of Y is taken to be the face surface.

OTHER ASPECTS

As explained in detail above, aspects of the invention are configured asdescribed above; but of course this invention is not limited to theseaspects. For example, the above-described crown has differentthicknesses in substantially the center portion and in the outerperipheral portion, but the entire crown portion may be of a smallerthickness than other body portions. Similarly, the thickness of thecenter area of the sole portion may be reduced, or the thickness of theentire sole portion may be made thinner than other body portions. Also,the materials comprised by the head are in essence all metal; but verysmall amounts of other materials can be used in some portions. Moreover,the numerical values stipulated by rules include tolerances, and ofcourse even when related numerical values fluctuate within the ranges oftolerances they remain within the technical scope of this invention.

1. A golf club, comprising: a face portion positioned on a front surfaceof a metal hollow golf club head and having a striking face to strike agolf ball; a crown portion forming an upper surface of the club, and asole portion forming a lower surface of the club, wherein the mass ofthe metal hollow golf club head is 210 g or less, characteristic time(CT value) of the metal hollow golf club head, relating to a restitutioncharacteristic, is 257 μs or less; and when a lie angle of the metalhollow golf club head is 60°, volume of the metal hollow golf club headis 470 cm³ or less, and moment of inertia about an axial line which isthe center of a plumb line passing through the center of gravity of themetal hollow golf club head, is in the range 5000 to 6000 g-cm².
 2. Thegolf club according to claim 1, wherein the metal is a titanium alloysheet member, and a substantial center portion and an outer peripheralportion including sites of the plumb line within the curved surface ofthe crown portion and/or the sole portion constituting the body differin thickness.
 3. The golf club according to claim 1 or claim 2, whereinthe metal hollow golf club head is formed by joining, by welding, theface portion, the sole portion, the crown portion, and a hosel portionto which the shaft is connected.
 4. The golf club according to claim 2,wherein a weight of 20 g or more is positioned at the position of arotation radius most distant from the plumb line and at the crownportion and/or the sole portion.
 5. The golf club according to claim 4,wherein the weight is positioned on a back side of the toe portion. 6.The golf club according to claim 5, wherein toe-side and back-side sitesof the sole portion are formed in shapes protruding outward relative tothe crown portion, and the weight is positioned in this protruding soleportion.
 7. The golf club according to claim 1, wherein the metal hollowgolf club head has: a length (X) from the heel portion to the toeportion longer than a length (Y) from the face portion to the rearsurface; a length from the heel to the toe of 127 mm (5 inches) or less;a length from the sole to the crown of 71.12 mm (2.8 inches) or less;and a moment of inertia (MOI) within a range calculated using thefollowing approximating equation (1):MOI=(aY ² +bY+c)×(dX+e)/f  (1) where X is the length from the heelportion to the toe portion, Y is the length from the face portion to therear surface, and a, b, c, d, e, and f are constants.
 8. The golf clubaccording to claim 1, wherein the metal hollow golf club head has: alength (X) from the heel portion to the toe portion longer than a length(Y) from the face portion to the rear surface; a length from the heel tothe toe of 127 mm (5 inches) or less; a length from the sole to thecrown of 71.12 mm (2.8 inches) or less; and a position of the center ofgravity, as seen from the plumb direction, existing within a rangeencompassed by the following two equations:Y=−gX ² +hX ² +i  (2)Y=−jX ² +k  (3) where X is the position in the direction from the heelportion to the toe portion, Y is the position in the direction from theface portion to the rear surface, g, h, j, and k are constants, the Xhas an origin at the center of the distance from the heel portion to thetoe portion, and the Y has an origin in the face surface.